1. Field of the Invention
This invention concerns those accelerometers in which the acceleration measurement is deduced from that of the forces required to hold a test weight still or bring it back to a position defined with respect to the body of the apparatus. It concerns more particularly those accelerometers where these forces are electrostatically produced and is intended to perfect them with a view to enhancing their accuracy.
2. Description of the Prior Art
U.S. Pat. No. 3,438,267 issued Apr. 15, 1969 described a capacitive microaccelerometer comprising a spherical conducting ball, relatively light, housed in a substantially spherical cage and freely floating in the cage where the diameters of the cage and the ball differ only very little. Inserted in the spherical cage are a number of electrodes insulated from the cage wall. There are twelve in all. Six of them are flat-dome shaped, polar electrodes arranged around poles where three tri-rectangular axes originating from the center of the spherical cage meet the cage wall and the other six are spherical rings surrounding and insulated from the spherical domes. The polar electrodes detect the position of the ball whilst the circumpolar electrodes control the positioning of the ball. The polar electrodes are fed by an alternating current source and, with the conducting ball when displaced by acceleration force effects, form a variable capacitor whose capacity depends on the ball position. Since the ball is free in the cage, its potential is capacitively fixed through a capacitor set up between its own surface and that of the cage wall not taken up by the polar and circumpolar electrodes; the capacity of this capacitor is relatively high and the impedance consequently low. The signals picked up between two given diametrically-opposed polar electrodes are detected, amplified and fed in the appropriate direction into the two circumpolar electrodes associated with the two given polar electrodes. A three-component ball position closed-loop control system is thus created wherein the signals applied to the three circumpolar electrode pairs respectively measure the components of acceleration applied to the ball.
In French Pat. No. 2 124 055 of Feb. 2, 1971, a polar electrode and two circumpolar electrodes, all coaxial, are arranged in the vicinity of each pole of the test ball. The nearest circumpolar electrode of the polar electrode assumes the position detection role; there are two positioning control electrodes per pole i.e. the polar electrode and the the circumpolar electrode farthest from the polar electrode. The positioning control signals are then no longer differentially applied between two circumpolar electrodes in relation to opposite poles but, depending on the direction of action, between the positioning control electrodes in relation to the same pole, or the opposite pole. This avoids, therefore, applying electrical charges to the floating ball when positioning it.
On a more general footing, the electrostatic return force generator is made up of a series of electrodes set out around the test weight such that the electrostatic forces that result from applying voltages or electrical charges to these electrodes give rise to a resultant in the opposite direction to the test weight displacement direction in the absence of these return forces.